With the progress of small-size, high-speed and lightweight electronic devices such as cellular phones and personal computers, the capacitor used for these electronic devices is demanded to have a smaller size, a larger capacitance and a lower ESR.
As an example of such a capacitor, the electrolytic capacitor has been proposed, which capacitor is produced by anodically oxidizing an anode body for capacitors comprising a sintered body made of a valve-acting metal powder such as tantalum which can be anodized to form a dielectric layer made of the oxide of the metal on the surface of the anode body.
The electrolytic capacitor using tungsten as a valve-acting metal and employing the sintered body of the tungsten powder as an anode body can attain a larger capacitance compared to the electrolytic capacitor obtained at the same formation voltage by employing the anode body of the same volume using the tantalum powder having the same particle diameter. However, the electrolytic capacitor having the sintered body of the tungsten powder has been unpracticed as an electrolytic capacitor due to the large leakage current (LC). In order to solve this issue, a capacitor using the alloy of tungsten and other metals has been studied and has achieved some improvement in the leakage current, but it was not enough (JP-A-2004-349658 (U.S. Pat. No. 6,876,083 B2); Patent Document 1).
Patent Document 2 (JP-A-2003-272959) discloses a capacitor using an electrode of a tungsten foil selected from WO3, W2N and WN2 having formed thereon a dielectric layer, but the capacitor is not to solve the above-mentioned leakage current problem.
Also, Patent Document 3 (WO 2004/055843 publication (U.S. Pat. No. 7,154,743 B2)) discloses an electrolytic capacitor using an anode body selected from tantalum, niobium, titanium and tungsten, but it does not describe a specific example using tungsten in the specification.